1. Field of the Invention
This invention relates to the mechanical arts. In particular, it relates to instruments for measuring and controlling the flow of fluids, such as gases.
2. Discussion of Relevant Art
The measurement and control of the flow of gases is important in many industries. During the manufacture of semiconductors, for example, many of the processes require a precise reaction of two or more gases under carefully controlled conditions. Since chemical reactions occur on a molecular level, the control of mass flow is the most direct way to regulate the reactants.
There have been developed in the art a variety of instruments for measuring the mass flow rate of gases from below 5 standard cubic centimeters per minute (SCCM) to more than 500,000 SCCM. The prevalent design of such instruments requires that the flow of the gas be divided into two or more paths.
In a typical instrument, a small amount of gas is routed through a flow sensor assembly, where the mass flow is measured, while most of the flow is routed through a splitter section located in parallel with the flow sensor assembly. The flow sensor assembly contains a flow tube that carries two resistance thermometers on its outside surface. The flow tube has an inside diameter of only about 0.01 to about 0.05 inch. The resistance thermometers are formed by winding multiple layers of insulated sensor wire, generally having a diameter of about 0.0006 inches, around the outside of the flow tube to form coils. To reduce induced signals from electromagnetic waves, upstream and downstream sensor coils are wound in opposite directions of rotation.
The resistance thermometers form two legs of an electronic sensor bridge; the other two legs are fixed resistors. The sensor assembly must be carefully designed and manufactured so that the two resistance thermometers are as identical as possible in electrical and thermal characteristics.
When a voltage is applied across the bridge, current passes through the resistance thermometers causing them to self-heat. Since they are nearly identical in electrical and thermal characteristics, the temperature of each resistance thermometer increases by the same amount, causing the electrical resistance in each to increase by the same amount--for so long as there is no gas flow through the flow tube. As soon as gas flow occurs, the upstream sensor wire is cooled by heat transfer to the flowing gas and the downstream sensor wire is either heated or cooled to a lesser extent. The average temperature of the upstream resistance thermometer will now be different than the temperature of the downstream resistance thermometer and the electrical bridge will show an imbalance due to the difference in their resistance.
So long as the flow sensor is not affected by external influences, the temperature difference between the resistance thermometers is only due to the mass flow and the specific heat of the gas. Therefore, if the specific heat is known, the mass flow can be directly measured.
Speed of response is a desired quality in flow measurement and control. Presently, it takes sensor assemblies from about one to eight seconds to respond to within two percent of a steady state value when the flow rate of a gas is changed. Although flow meters can be made to operate more quickly by use of electronic signal conditioning, ultimately it is the flow sensor assembly which limits the speed of the system.
Another desired quality in mass flow measurement and control is linearity. A linear mass flow system greatly simplifies interaction with an instrument's automatic process control systems. The sensor assemblies which are commercially available have a typical nonlinearity effect of approximately three to five percent variation from an ideal straight line response.
A critical and failure-prone area of sensor assemblies is the termination of the very fine sensor wire to a much larger wire in order to make electrical connection and form the electronic sensor bridge. The problem is compounded by the fact that thermal inertial masses, such as large lead wires or terminal connectors, attached to the sensor must be kept to a minimum in order not to slow the response and reduce the electrical output of the sensor assembly. This termination is handled in a variety of ways in existing flow meters. Unfortunately, all of the ways involve problems that result either by having an imbalance created between the two resistance thermometers or by requiring excessive thermal mass, which slows the sensor assembly's response.
Free convection currents can create attitude sensitivity problems when conventional sensor assemblies are tilted, so that one sensor is higher than the other. Convection currents are caused by warmer gases near the flow tube rising due to buoyancy forces. When the flow sensor assembly is horizontal, these warm currents rise at right angles to the flow tube and do not circulate axially along the flow tube. However, when the flow sensor assembly is tilted, these warm currents have an axial component, relative to the flow tube, and the rising warm gas transfers heat from the lower resistance thermometer to the higher resistance thermometer. This heat transfer results in a temperature differential between the two thermometers and gives a false indication of flow through the flow tube.
Conventional flow meters have attempted to block free convection currents on the outside of the flow tube by encapsulating the resistance thermometers in a foam material of low thermal mass. Unfortunately, the addition of the foam material in contact with the resistance thermometers reduces the thermal resistance between the resistance thermometers and the surrounding environment. As a result, the sensor assembly's operating temperature and output signal is reduced. In addition, over a period of time, the thermal resistance of the foam can change due to thermal expansion or contraction, chemical changes or inertial effects. These changes result in long-term drift in the output. Moreover, the foam material increases the thermal mass adjacent to the flow tube. This increase in thermal mass slows the flow sensor assembly's speed of response to internal thermal changes due to different mass flow rates.
Another attempt at minimizing attitude sensitivity in mass flow meters involves using a single housing, made of metal or material having a relatively high thermal conductivity, spaced closely to the sensors. Unfortunately, the housing does not completely eliminate the problems which can arise because of free convection currents.
Additionally, electromagnetic waves can induce electrical currents of considerable strength in the flow tube and resistance thermometers. Problems arise, when these elements interact with such electromagnetic waves and act as transformers creating voltages across the sensor bridge that are not related to gas flow.